Yes, Elena, I'm a vampire. Klaus is an original vampire. He used your blood in a sacrifice to make himself a hybrid. He tricked us into to believing it was another curse. But we managed to find a way to bring you back. Then he compelled me to switch off my humanity after making a deal with me for a cure for my brother that required his blood. He's good at compelling people to do what he wants. He killed Tyler to make Bonnie figure out a way to make his line of hybrids not die.